The pre-and post-synaptic elements of synaptic junctions are linked together by cell adhesion molecules: proteins that play crucial roles in stabilizing, signaling and adjusting the strength of the synapse. The cadherins, 1 type of adhesion molecule, link together 2 adjacent cell membranes by forming homodimers in 1 membrane that interact across the junction (synaptic cleft) with homodimers formed in the opposing cell membrane. In the last decade, much effort has been devoted to understanding the structure and molecular associations of the classic cadherins. Crystallographic and biophysical studies have yielded somewhat conflicting results and, as yet an unclear, picture of the interactions of the cadherin extracellular domain during dimerization. To better understand the dynamics of cadherin interactions we are developing Fluorescence Resonance Energy Transfer (FRET) based sensors to monitor cadherin associations across cellular junctions. FRET is unique in its ability to provide signals that are sensitive to changes in intra-or intermolecular distances in the 1-10 nm range, well below the inherent diffraction limit of conventional fluorescence microscopy. Cadherins will be fluorescently tagged in their extracellular domains; FRET-donor and FRET-acceptors will be expressed in pre-and postjunctional cells and the strength of adhesion will be monitored. It has long been known that and the structural integrity of cadherins is dependent on the local Ca2+ concentration. In the absence of Ca2+, cadherins undergo a reversible loss of their rod-like structure and collapse. Experimental data as well as simulations predict that Ca2+ is dynamically regulated in the synaptic cleft. We predict that alterations in cleft Ca2+ have important ramifications for cadherin-cadherin adhesion and signaling. We will use FRET to monitor dynamically, in living cells, cadherin interactions and conformational changes induced by changes in extracellular calcium and synaptic activity. By virtue of their localization at synaptic cleft and their interactions with cytoplasmic proteins, like beta-catenin, the cadherins occupy a pivotal position that can contribute to the synaptic dysfunction associated with disease. For example, in the absence of presenilin 1, a protein mutated in some forms of Alzheimer's disease, cadherins become destabilized at adherent junctions and fail to localize properly. This mislocalization presumably leads to profound alterations in synaptic structure and function. [unreadable] [unreadable]